rhodium wires with corresponding galvanometer-pyrometer (Fig n p G)
^ (6) Two U-tubes, one filled with pumice and chromic acid and the other
with calcium chloride and phosphoric anhydride.
(7) The usual absorption apparatus for the carbon dioxide (see preced-
ing method), followed by a sulphuric acid wash-bottle.
Procedure. When the
apparatus is arranged, a
moderate current of oxygen
is passed for about 10
minutes and then, complete
elimination of the air being
ensured, without interrupt-
ing the flow of gas the
absorption apparatus is de-
tached and weighed when
it has reached the tempera-
ture of the surrounding air.
Meanwhile the furnace is
heated and the sample
weighed in an unglazed por-
celain boat (10-12 cm. long,
1-4 cm. wide). Of malleable
iron or ordinary cast-iron, i
gram is taken, or of iron
alloys rich in carbon, such
as ferro-manganese, ferro-
chromium, etc., 0-5 gram. The sample should not be too finely pow-
dered in order that too violent combustion may be avoided.
When the furnace reaches a temperature of about 900°, the stream of
oxygen is interrupted and the boat pushed exactly into the middle of the
tube by means of a copper wire or a quartz rod fitted with a hook, the tube
being then quickly closed and the absorption apparatus attached.
A fairly rapid current of oxygen is then passed and the temperature
of the furnace raised to between 1050° and 1150° in 15-20 minutes. The
latter temperature should not be exceeded, since otherwise the furnace
may be damaged and the sample may begin to fuse and enclose a little
carbon, which may thus escape complete combustion. The temperature
is kept at 1050-1150° for 15-20 minutes, the heating being then interrupted
and the velocity of the oxygen increased until about 700° is reached, when
the absorption apparatus is detached and, 15-20 minutes later, weighed.
The weight of the carbon dioxide, multiplied by 0-27273, gives the carbon
in the sample taken. After removal of the boat the apparatus is ready for
the next determination.
In determining carbon in certain ferro-metallic alloys, such as ferro-silicon,
ferro-chromium and ferro-manganese, it is necessary to add a catalyst to ensure
the complete oxidation of the carbon. According to Ledebur, the best results
are obtained by cobalt oxide, which should be calcined for half an hour in an
electric furnace at about 1000° before use. One gram of the oxide is mixed in
FIG. iiontaining' sodium hydroxide solution ami cone,